In recent years, the cognitive radio technique has been attracting attention in order to effectively use stringent radio frequency bands. Examples of typical cognitive radio standards include IEEE 802.22. The cognitive radio technique according to the IEEE 802.22 searches for an available frequency band (white space) from among the frequency bands for television, which extend from 54 MHz to 862 MHz, and uses the available frequency band for a secondary purpose. The implementation of the cognitive radio technique requires a frequency component analysis apparatus for searching for a white space from among a wide range of frequency bands.
If the frequency component analysis apparatus can find a white space at a high speed, the radio apparatus can be used for a longer time, which is longer because the white space can be found in a shorter time, for data communication. Further, if the frequency component analysis apparatus can immediately detect the presence of a radio system(s) having a priority (a television set in the above-described example), the radio apparatus can prevent any interference with the system having a priority by using the cognitive radio system. Therefore, it is desired that the frequency component analysis apparatus, which searches for a white space, has a high-speed characteristic.
Patent literature 1 discloses a frequency component analysis apparatus including a local oscillator using a phase-locked loop, a mixer for performing a frequency conversion by multiplying a signal to be analyzed by a local oscillation signal, a filter for removing a unnecessary frequency component, and an intensity detector for detecting the intensity of a signal contained in a necessary frequency band. The frequency component analysis apparatus like this sweeps local oscillating frequencies in a range of a necessary frequency band and thereby can analyze a radio-wave intensity distribution in the swept frequency band by using a frequency resolution determined by the band width of the filter.
However, it is very difficult to increase the processing speed for the frequency component analysis by using the simple configuration like this. That is, in the method in which local oscillating frequencies are swept in a predetermined frequency range while locking the frequency with a predetermined resolution in a stepwise manner, the sweeping speed depends on the lock time that is determined by the time constant of the phase-locked loop. Specifically, while the convergence time of the filter used for extracting only the signal band of the digital television broadcast, whose signal band width is 8 Mhz, and the convergence time of the intensity detector for the frequency component that has passed through the filter are no greater than 10 microseconds, the lock time for one frequency is about 50 microseconds to 100 microseconds.
Further, in measurement apparatuses for frequency component analysis, in general, a ramp signal is added to the control value of a variable frequency oscillator while locking a local oscillating frequency at the center of a predetermined frequency range. The method for sweeping frequencies like this has an excellent high-speed characteristic. However, this method requires a circuit for generating a ramp signal in addition to the phase-locked loop. In general, an operational amplifier is used for the ramp signal generation circuit. Therefore, there are problems that the power consumption is large and a number of capacitive elements are used, requiring a large area. Further, there is another problem that when the relation between control values and local oscillating frequencies is not linear, a large frequency error occurs.
Therefore, Patent literature 2 and 3 disclose configurations in which a frequency oscillator receives digital data, generates a frequency according to the digital data, and outputs the generated frequency. The digital data and the frequency to be generated are associated with each other in advance. This configuration eliminates the need for outputting a signal or the like to the frequency oscillator by using the phase-locked loop.